The basic function of a yarn clearer (yarn quality sensor) on a yarn making textile machine is detecting yarn defects during yarn production. Nowadays, the evaluation of whether a yarn parameter which is detected in real-time during yarn production by the clearer (yarn quality sensor) is to be considered a defect is basically carried out by three different methods.
The first method is the so-called absolute method, in which a current yarn parameter, such as thickness, unevenness, is determined, e.g. by measurement or calculation, and this parameter is compared in an absolute manner to a set defect level. If the determined parameter or absolute deflection of the determined parameter, i.e. the absolute value of the difference between the measured and desired (set) parameter, exceeds the set limit (set fault level), this is assessed as a yarn defect and, if appropriate, subsequent measures are taken to remove the defect.
The disadvantage of the absolute method is the fact that it cannot be used for those defects for which the correct or desired absolute value of the parameter is not known, for which a relative change in the parameter is evaluated, or this value is dependent on the particular workstation at which yarn is produced. Typically, such defects include long defects of yarn (sliver defects, defects related to the yarn count or more).
The second method is the so-called relative group method, in which the yarn parameter measured or calculated is compared to the mean value of the same yarn parameter measured by a group of clearers (yarn quality sensors) or it is compared to the parameter calculated for a group of clearers (yarn quality sensors). The group of clearers (yarn quality sensors) is on the same machine and the workstations which produce the same yarn. If the difference or relative deflection between the measured or calculated parameter of the clearer (yarn quality sensor) being evaluated and the mean value of the same parameter calculated from the data of the above-mentioned group of clearers (yarn quality sensors) exceeds the set limit, this state is assessed as a yarn defect and, if appropriate, subsequent measures are taken to remove the defect.
The drawback of using the relative group method is the fact that this evaluation is not appropriate for defects for which even on yarn without defects too broad a range of the values of the particular parameter is recorded at the individual workstations. In extreme cases, using the relative group method leads to the difference between the correct and defective value of the yarn parameter at one workstation is smaller than the variation range (maximum-minimum) of this parameter for a group of workstations. Such typical defects are yarn unevenness and hairiness.
The third method is the so-called relative individual method, in which the measured or calculated parameter of each clearer (yarn quality sensor) in real-time is compared to the “correct”, i.e. reference value of the parameter of the particular workstation. If the difference between the measured and the individual reference value of the particular workstation, or a relative deflection of the determined parameter from the individual reference value of the particular workstation exceeds the set limit, this state is assessed as a yarn defect and, if appropriate, subsequent measures are taken to remove the defect. The above-mentioned “correct”, i.e. reference value is obtained either by calculation or by measuring the correct, i.e. reference yarn at the particular workstation, at which yarn is produced and which applies this “correct”, i.e. reference value for cleaning production yarn or for monitoring the quality of production yarn. At present, in most cases this type of individual reference parameter is determined when first starting the workstation with new operating values (yarn count, twist, etc.) at which the parameter is evaluated, or the reference parameter is determined each time the workstation is started, after the first spinning-in operation on an empty tube and, in case of need, corrections are made to the reference parameter even during the ongoing production of yarn.
The disadvantage of the relative individual method consists in determining the reference value of the parameter, to which the measured or calculated parameter is to be compared. It is the determination of the reference value of the parameter that is a critical moment. The main reason is that there is a considerable risk that during the determination of the reference value, i.e. during measurement of the parameter for the determination of the reference value, defective yarn is produced, which, however, cannot be classified as defective, since the system of the clearer (yarn quality sensor) actually “does not know” that it is measuring defective yarn, since it does not have the reference value (which is being determined during this measurement process), against which it makes a decision as to whether the yarn being produced is faultless or defective. Such typical defects include, for example, defects caused by a failure of the moistening system of yarn during yarn production on an air-jet spinning machine, when a lack of water manifests itself in increased yarn hairiness, which has no effect on the yarn clearer or the sensor of yarn quality and therefore such a defect cannot be automatically detected. A similar situation may occur when the pressure drops in the spinning nozzle during the production of yarn on an air-jet spinning machine.